FIG. 1A shows a typical fiber optic sensor 1 for use in an inline time division multiplexed (pulsed) array. Sensor comprises a fiber optic sensor coil 10 and a reference line 12, each formed from an optical fiber cable generally denoted 22, disposed between a pair of couplers generally denoted 14. Typically the reference line 12 is physically isolated from perturbation by external factors. Each coupler 14 typically is a fiber optic coupler and has two input ports and two output ports. A pulsed coherent light source 18 supplies a series of light pulses to sensor 1. To interrogate sensor 1, a compensating interferometer or compensating sensor 26 is required between the output of sensor 1 and a photodetector 20.
Alternatively, as shown in FIG. 1B, a sensor 1' comprises a coil 10 coupled between a first output port of a coupler 14 and a discrete reflector or mirror 24a. A second reflector or mirror 24b is coupled to the second output port of coupler 14. Reflectors 24a and 24b are full reflectors reflecting all of the impinging coherent light. The configuration of FIG. 1B is functionally identical to that of FIG. 1A.
An illustrative example of sensor 1 operation will now be discussed with reference to FIG. 1A. During operation, coherent light source 18 is coupled to both coil 10 and reference line 12 via coupler 14a and a fiber cable 22a. Sensor 1 responds to a measurand, such as acoustic waves impinging on sensor coil 10, by changing the length of the coil 10 as a function of the magnitude of such measurand. The compensating interferometer 26 is responsive to the optical output of coherent light produced by coil 10 for developing an optical interference pattern, which is detected by a photodetector 20. Photodetector 20 generates an electrical signal in response to the optical interference pattern, thereby providing an electrical signal representative of the impinging acoustic wave.
The use of a plurality of fiber optic acoustic sensors as described hereinabove in an inline array of such sensors is also known. As illustrated in FIGS. 2A-2D, a plurality of sensors 1 and 1' (FIGS. 1A and 1B) are combinable in various series arrangements to form inline fiber optic acoustic sensor arrays 2 through 5, respectively. FIG. 2A shows a Fabry-Perot array 2 formed from a plurality of sensors 1'. Coupler 14 and reflector 24b of sensor 1' (FIG. 1B) are replaced by a partial reflector 24', as shown, thereby reducing the number of elements needed to form an inline array of sensors 1'. FIG. 2B shows a tapped serial array 3 formed by a plurality of sensors 1 as disclosed in U.S. Pat. No. 4,889,986. FIG. 2C shows a Stanford ladder array 4 produced by an alternative configuration of sensor 1, while FIG. 2D shows an inline Michelson array 5 produced by an alternative configuration of sensor 1'. It will be appreciated that a complete sensor is defined between each adjacent pair (set in the case of a Stanford ladder array) of couplers or reflectors.
Fiber cables 22 employed in forming sensing coils 10 are sensitive to a large number of environmental effects, such as temperature fluctuations and pressure variations. It is known to provide a compensating interferometer 26, having the configuration of sensor 1 in FIG. 1A, in each of the arrays 2 through 5 to compensate for the path differences of individual sensors 1 and 1' in the arrays 2 through 5.
Referring to FIGS. 3A-3D, in accordance with conventional techniques for packaging arrays 2 through 5 into deployable assemblies, repeating segments of each array are packaged as identical sensor units 30 located along each array 2 through 5, respectively. For example, as shown in FIG. 4 with respect to array 3 of FIG. 3B, each sensor unit 30 in an inline array is conventionally formed by mounting together a sensing coil 10 wrapped on or embedded in a compliant medium 11, a reference line 12 and one of the couplers 14 (reflectors 24' in the case of the FIG. 3A array) associated with a given sensor 30. A housing 40 can be provided which encloses the sensing coil 10, reference line 12 and the associated coupler 14. A perforated aluminum tube from 5-10 inches long and approximately 11/2 inches in diameter is commonly used as housing 40. A potting medium (not shown) is used to secure reference line 12 and the coupler 14 within housing 40. Alternatively, instead of providing a separate housing for each sensing unit, the entire array is typically disposed within a protective hose or other tubular member 23 for deployment. Thus, each sensor unit 30 so packaged contains only a portion of a complete functional sensor (sensor 1 shown in FIG. 1A for the FIG. 3B array, and sensor 1' shown in FIG. 1B for the FIG. 3A array).
The arrays 2-5 shown in FIGS. 3A-3D, respectively, are all used with conventional signal processing circuitry such as that disclosed in U.S. Pat. No. 4,889,986, wherein sensor responses are determined based on the travel time of coherent light pulses through each sensor of a sensor array. However, as noted above, inline sensor arrays 2-5 with sensor unit packaging as shown in FIGS. 3A-3D do not include clearly defined sensors.
In addition, the required spacing between sensor units 30 in the arrays 2-5 necessitates long leads on coils 10 and reference lines 12. This further complicates measurement of the signals of interest from sensors 1, 1', because environmental stresses produced in the portions of fiber cables 22 connecting the sensor units to each other are indistinguishable from the stresses produced within the sensor units in response to the acoustic pressure waves of interest.
Heretofore, an improved inline sensor array which decouples environmental stress in the portions of fiber cables 22 connecting the sensor units to each other in the array has not been achieved.